Asynchronous transfer mode (ATM) is a protocol-independent, cell-switching technology that offers quality of service guarantees for the support of voice, data and video traffic. ATM uses a layered protocol model. The ATM Adaptation Layer (AAL) provides the necessary services to support the higher-level protocols and can exist in end stations or in network switches of a link or network. This layer assures a number of tasks including segmenting the information into 53-byte cells and reassembling it back into its original format. Several different AAL protocols have been defined for different applications. The asynchronous transfer mode adaptation layer 2 (AAL2) protocol was first defined in the ITU-T Recommendation I.363.2 in 1997 and is the most favourable of the protocols for supporting low and variable bit rate (LBR/VBR) voice-over-ATM. Variable bit rate (VBR) services enable statistical multiplexing for the higher layer requirements demanded by voice applications, such as compression, silence detection and/or suppression, and idle channel removal. Moreover, AAL2 enables multiple user channels on a single ATM virtual circuit or virtual channel connection (VCC) and varying traffic conditions for each individual user or channel. The structure of AAL2 also provides for the packing of short length packets (or packet segments) into one ATM cell. This protocol thus offers a variable payload within cells and across cells which provides good bandwidth efficiency. For these reasons, AAL2 is the most suitable protocol for voice traffic, since it provides low packetisation delay which enables an operator to provide adequate quality of service to the user, and also allows the efficient use of bandwidth, which is of economic importance to many telephone operators who have leased lines in their networks.
All AAL protocols define a cell header that includes routing information in the form of a virtual path (VP) and virtual channel (VC) identifier. The AAL2 permits information from several channels to be carried in a single ATM cell. To this end the cell payload may contain multiple AAL2 packets, each of which has a packet header including a ‘connection identifier’ (CID) that identifies the individual AAL2 channels. The CID consists of 8 bits, so theoretically 256 different AAL2 user channels could be multiplexed in a single ATM virtual channel connection (VCC). In practice this number is reduced to 248 since eight identifiers are reserved. When a number n of ATM virtual channel connections are used identified by the VPI/VCI combination, the total number of user channels available is n×248. A problem exists when these channels must be processed before reaching the final end destination. For voice applications this processing may involve the compression and decompression of channels to optimise bandwidth utilisation. Providing a resource for compression and decompression for each channel is costly, inefficient and often physically impossible. However, when fewer resources than the total number of user channels are available, some form of mapping is required. Presently available applications use fixed mapping between the VP/VI and CID and resource identity. Fixed mapping may keep delays to a minimum and permit a relatively compact hardware implementation, however, changes in the network may mean that it is impossible to make the most efficient use of all resources.
It is thus an object of the present invention to present a method and arrangement for permitting the dynamic mapping of user channels or other packet-switched data to resources.
It is a further object to provide a method and arrangement of dynamic mapping of user channels or other packet-switched data to resources that imposes only a minimal delay on processing, and preferably that is both compact and easy to implement.